Austenitic ductile irons are well known and have been used for many years in a wide range of applications requiring materials with specific chemical, mechanical, and physical properties. Ductile cast irons are cast irons containing graphite in the form of substantially spheroidal particles produced by suitable molten metal treatment. Spheroidal graphite has a polycrystalline radial structure. There are a number of different austenitic ductile iron types defined in the literature. The austenitic ductile iron types are defined based in part on their chemical makeup, comprising various amounts of iron, nickel, silicon, and carbon and, in some iron types, additional elements such as manganese, phosphorus, chromium, and molybdenum. These latter elements may be intentionally added or may be present as unavoidable impurities. The austenitic ductile iron types are further defined based on their varying levels of mechanical properties (i.e., tensile strength, yield stress, elongation, and Brinell hardness). In general, austenitic ductile irons typically display good corrosion, erosion, and wear resistance; good strength, ductility, and oxidation resistance at high temperatures; toughness and low temperature stability; controlled thermal expansion; controlled magnetic and electrical properties; and good castability and machinability. However, these qualities vary, depending on the type of austenitic ductile iron and accordingly, specific types are more useful than others in certain applications.
Austenitic ductile irons are commonly used in engine components such as exhaust manifolds, turbine housings, and other structural components that must operate under high thermal stress. Demand for improvements in fuel efficiency and reduction in exhaust gas from automobile engines has been satisfied by increasing engine power and combustion temperatures. These increases put a greater strain on structural engine components due to the increased temperature of exhaust gases that must pass through them. Specifically, the material used to construct the engine components must have high temperature resistance, good temperature fluctuation resistance, high scaling resistance, and low temperature expansion coefficient.
The increased temperatures at which engine components must operate has resulted in a more limited range of materials that can be used to construct such engine components. The austenitic ductile iron most commonly used to cast structural engine components is D-5S austenitic ductile iron. In particular, D-5S iron is commonly used in engine manifolds, turbine housings, and turbocharger components where high temperatures and severe thermal cycling occur. According to a widely accepted standard (ASTM A439), D-5S comprises 2.3% carbon, 1% manganese, 4.9-5.5% silicon, 1.75-2.25% chromium, 34-37% nickel, 0.08% phosphorus, with the balance iron. This alloy exhibits good elongation and yield strength at room temperature, good castability, and relatively good high temperature yield strength at exhaust gas temperatures up to about 900° C.
D-5S is highly alloyed, commonly comprising around 36% nickel. Nickel is an expensive raw material and suffers from a large fluctuation in price, which has become increasingly volatile. The high cost of nickel directly impacts the cost of the end product. Because the turbocharger housing and, in particular, the turbine housing of a turbocharger comprises the greatest weight, it also comprises the greatest cost of the entire turbocharger. Thus, the cost of the end product must be dramatically increased to account for the high cost of the nickel.
Accordingly, it would be beneficial to produce an alternative alloy for use in such structural components that is capable of withstanding the high temperatures required by modern engines, while minimizing the amount of nickel to keep costs low and more predictable.